Source : Webster's Revised Unabridged Dictionary (1913)

Arrhythmic \Ar*rhyth"mic\, Arrhythmous \Ar*rhyth"mous\, a. [Gr.
   ?; 'a priv. + ? rhythm.] (Med.)
   Being without rhythm or regularity, as the pulse.

Source : WordNet®

arrhythmic
     adj 1: without regard for rhythm [syn: {arrhythmical}]
     2: not having a steady rhythm; "an arrhythmic heartbeat" [syn:
        {jerking}, {jerky}, {unsteady}]